“There were times when, some weeks … I haven’t counted exactly, but I would just sort of sleep for a few hours, work, sleep for a few hours, work, seven days a week. Some of those days must have been 120 hours or something nutty.”

Now, Musk said he is “down to 80 or 90” hours of work per week and “it’s pretty manageable.”

Musk said while “80 is pretty sustainable,” the increase in suffering when you work more than that is dramatic. “The pain level for hours increases exponentially. It’s like nonlinear above 80,” Musk said, according to a transcript.

Still, 80 hours a week is over 10 hours more than the average CEO of a multibillion-dollar company works.

A study published in June by Harvard Business Review found CEOs worked an average of 62.5 hours a week. On average, the CEOs monitored for the study worked 9.7 hours per weekday, which totals just 48.5 hours per work week. They also worked 79 percent of weekend days at an average of 3.9 hours daily, and 70 percent of vacation days with an average of 2.4 hours on those days.

For the study, Harvard professors Michael Porter and Nitin Nohria, tracked how 27 CEOs of companies with an average annual revenue of $ 13.1 billion spent their days in 15 minutes increments, 24 hours a day, seven days a week for three months.

As for Musk, he has attributed some of his recent controversial behavior to his work schedule.

“Obviously I think there’s really no excuse for bad manners and I was violating my own rule in that regard. There are reasons for it, I got no sleep, 120 hour weeks, but nonetheless, there is still no excuse, so my apologies for not being polite on the prior call,” Musk said.

Later in August, in conversation with the New York Times, Musk reported using prescription sleep medication Ambien to sleep.

“Yeah. It’s not like for fun or something,” Musk told Swisher Wednesday. “If you’re super stressed, you can’t go to sleep. You either have a choice of, like, okay, I’ll have zero sleep and then my brain won’t work tomorrow, or you’re gonna take some kind of sleep medication to fall asleep.”

Musk said he was working such insane hours to get Tesla through the ramp up in production for its Model 3 vehicle. “[A]s a startup, a car company, it is far more difficult to be successful than if you’re an established, entrenched brand. It is absurd that Tesla is alive. Absurd! Absurd.”

Sleep advocate and founder of Thrive Global Arianna Huffington wrote an open letter to Musk in August encouraging the founder to get more sleep.

“Working 120-hour weeks doesn’t leverage your unique qualities, it wastes them,” Huffington said. “You can’t simply power through — that’s just not how our bodies and our brains work. Nobody knows better than you that we can’t get to Mars by ignoring the laws of physics. Nor can we get where we want to go by ignoring scientific laws in our daily lives.”

In response to Huffington’s letter, Musk tweeted: “You think this is an option. It is not.”

But Musk did admit to Swisher that the long hours were not good for him.

“Pretty sure I burnt out a bunch of neurons during this process. Running both SpaceX and Tesla is incredibly difficult,” Musk said. “This year felt like five years of aging, frankly. The worst year of my entire career. Insanely painful.”